DESCRIPTION OF THE FOSSILS. 
219 
S. integricosta. Ph. PI. X. fig. 2. Bolland ; Northumberland. 
Beaks very approximate ; radiating ribs very obtuse, entire. 
S. planata. Ph. PI. X. fig. 3. Bolland. 
Upper valve nearly plane ; radiations many, obtuse. 
S. ovalis. Ph. PI. X. fig. 5. Bolland. 
Mesial fold obtusely rounded ; lateral plaits broad. 
S. trisulcosa. Ph. PI. X. fig. 6. Bolland. 
Smooth, oblong ; one mesial and two lateral ridges on the upper valve. 
S. triradialis. Ph. PI. X. fig. 7- Bolland. 
Smooth, orbicular ; one mesial and two lateral ridges on the upper valve, which is 
flattened. (Analogous to Sp. planata.) 
S. sexradialis. Ph. PI. X. fig. 8. Bolland. 
Oblong, one mesial and six lateral ridges on the upper valve. 
S. linguifera. Ph. PI. X. fig. 4. Bolland. 
Oblong, convex, without angles ; mesial fold rounded, prominent ; lateral radiations 
obscure. 
D. Glabratee. Cardinal area not so wide as the shell ; surface not ra- 
diated. 
S. decora. Ph. PI. X. fig. 9. Bolland. 
Nearly orbicular, convex, beak prominent ; faintly radiated ; mesial fold indistinct, 
partially divided. 
S. glabra. Mart. Sow. PI. X. fig. 10, 11, 12. Bolland ; Arran ; Ire- 
land ; Isle of Man. 
A variable species. Smooth ; length from J to § of the width : gibbous, rounded ; 
the mesial fold very large and prominent, round or flattened in the middle ; lateral 
radiations obscure or none. (I unite Sp. glabra and Sp. obtusa of Sowerby.) 
S. symmetrica. Ph. PI. X. fig. 13. Bolland. 
Rotundato-quadrate, gibbous, smooth ; mesial fold wide, (sometimes divided in the 
middle.) 
S. mesoloba. Ph. PI. X. fig. 14. Bolland. 
Depressed, surface imbricate, mesial fold distinct. 
S. lineata. Mart. Sow. PI. X. fig. 17- Kirby Lonsdale ; Crooklands ; 
N orthumberland. 
Surface imbricate ; no mesial fold ; convex. 
S. elliptica. Ph. PI. X. fig. 16. Bolland ; Queen’s County, Ireland. 
Mesial fold obtuse ; surface radiated, and concentrically striated. (The concentric 
striae vanish with age.) 
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